1. Field of the Invention
The present invention relates to hydraulically powered equipment, such as off-road construction and agricultural vehicles, and more particularly to apparatus for reducing oscillation or wag when a hydraulically driven member on the equipment is decelerating, stopping, or reversing direction.
2. Description of the Related Art
With reference to FIG. 1, a backhoe 2 is a common type of earth moving equipment that has a boom assembly 3 comprising a bucket 4 attached to the end of an arm 5 which in turn is coupled by a boom 6 to the frame of a tractor 7. Three hydraulic cylinders 11 form actuators that are operated independently to move the bucket, arm, and boom. A pivot joint 8 allows the boom assembly 3 to swing left and right with respect to the rear end of the tractor 7. A hydraulic boom swing cylinder 9 is attached to the boom 6 on one side of the tractor 8 and provides the driving force that swings the boom assembly. On larger backhoes, a pair of hydraulic cylinders are attached on opposite sides of the tractor 7 to swing the boom. Hydraulic fluid is supplied to the boom swing cylinder 9 through valves that are controlled by the backhoe operator.
As the boom swings in one direction, pressurized fluid is introduced into one chamber of the boom swing cylinder 9, designated as the “driving chamber”, and fluid is exhausted from the other cylinder chamber, referred to as the “exhausting chamber”. When the boom swings in the opposite direction, the designation of the driving and exhausting chambers is reversed. When the operator suddenly stops the boom swing, inertia causes the motion of the backhoe boom assembly 3 to continue in the previously commanded direction. The amount of inertia is a function of the mass and extension position of the boom assembly 3 and the mass of any material carried in the bucket 4. This continued movement due to inertia compresses the hydraulic fluid in the previous exhausting chamber of the boom swing cylinder 9 and may produce cavitation in the previous driving cylinder chamber. Anti-cavitation valves typically are provided in the hydraulic system to overcome this latter problem.
Because the control valves for the cylinder are now closed, pressure in the previous exhausting chamber eventually increases to a magnitude that causes the boom assembly 3 motion to stop and recoil by moving in the opposite swing direction. This subsequent movement produces a reversal of the pressure conditions, wherein the previous driving chamber of the boom swing cylinder 9 becomes pressurized. When the boom motion in the opposite swing direction creates a sufficiently high pressure in the previous driving chamber, another reversal of the swing motion occurs. As a result, the boom assembly swing oscillates until inherent dampening provided by other forces ultimately brings the assembly to a stop. This phenomenon is known as “bounce” or “wag” and increases the time required to properly position the boom 6, thereby adversely affecting equipment productivity. The wag also is disconcerting to the machine operator. A similar motion phenomenon occurs when other types of hydraulically driven members stop.
In essence, the wag is a manifestation of the inability of the boom velocity to promptly respond to, or track, changes in the position of the valve that controls the flow of fluid to the swing cylinder. In other words, the valve closes when motion of the boom is to terminate, however the load force acting on the boom does not allow the velocity of the boom to decrease fast enough.
Various approaches have been utilized to minimize this wag. For example, U.S. Pat. No. 4,757,685 employs a separate relief valve for each hydraulic conduit connected to the swing cylinder chambers to vent fluid to a tank line when excessive pressure occurs in the associated chamber. Additional fluid is supplied from the supply line through makeup valves to counteract cavitation in the cylinder as the swing stops.
U.S. Pat. No. 5,025,626 describes a cushioned swing circuit which also has relief and make-up valves connected to the hydraulic lines for the boom swing cylinder. This circuit also incorporates a cushion valve which in an open position provides a fluid path between the cylinder hydraulic lines. That path includes a flow restriction orifice. The cushion valve is resiliently biased into the shut position by a spring and a mechanism opens the cushion valve for a predetermined time period when the pressure differential between the cylinder chambers exceeds a given threshold. Both of these previous solution attempts required additional valves and other components.
U.S. Pat. No. 6,705,079 describes another solution to the swing wag problem in which a sensor detects pressure in the hydraulic actuator. This pressure signal from the sensor is employed to determine the rate at which the pressure in the hydraulic actuator changes. When the rate of change of the pressure is less than a defined threshold after receiving a stop command, pressure in the hydraulic actuator is relieved, such as by opening a control valve connected to the hydraulic actuator.
However, there still is a desire to improve the responsiveness of the boom velocity to changes in the position of the valve and the resultant flow of fluid to the associated hydraulic cylinder, and in particular to provide a simplified mechanism for reducing wag.